Our understanding of the complexities of pathogenic mechanisms of HIV disease is still evolving. This project has been investigating the immunologic consequences of HIV envelope glycoprotein mediated ligation of CD4 molecules. The underlying hypothesis is that HIV can lead to aberrant CD4 signaling which constitutes a key pathogenic mechanism in HIV infection, leading to aberrant cytokine secretion and contributing to immunologic anergy, quantitative loss of lymphocytes by apoptosis and virus upregulation. During the previous funding period we demonstrated that simple CD4 ligation by HIV gp120 or by anti-CD4 antibody resulted in a state of functional unresponsiveness in CD4+T cells, whereas if the CD4 molecules were crosslinked (XL), the cells were primed from apoptosis. Importantly we demonstrated that CD4 crosslinking of normal peripheral blood mononuclear cells led to induction of Fas antigen upregulation and apoptosis in CD4 T cells. The latter process was associated with the induction of cytokines TNF-alpha and Interferon-gamma (without IL-2 or IL- 4). We propose to continue these studies with the aim of understanding the immunologic and virologic effects of CD4 ligation in HIV disease pathogenesis. Our specific aims are to elucidate the mechanisms oc CD4 crosslinking (CD4XL)-induced T cell apoptosis with the goal of studying interactions and relationship between the apoptosis promoting molecules. Fas and Fas-ligand (Fas-L). and the apoptosis repressor system made up of the Bcl-2 proteins, the role of monocyte/macrophages in the induction of T cell apoptosis, the regulatory role of cytokines in this process and finally to investigate intracellular signaling that is associated with Fas'Fas-L interaction. We also observed that CD4XL in patient cells leads to HIV upregulation. A critical question being asked is the mechanism by which immunologic forces drive replication. Studies in patients will test the proposed model of CD4XL-induced induction of apoptosis and HIV upregulation. Attempts will be made to reverse or prevent CD4XL-induced harmful effects. These findings have relevance for understanding AIDS pathogenesis and for designing immune based therapeutic strategies and vaccines.